1. Field of the Invention
This invention relates to a semiconductor integrated circuit, and particularly to an improvement for stabilizing a voltage at a power supply wiring.
2. Description of the Related Art
Recent semiconductor integrated circuits have a great number of logic gates on a single semiconductor chip. Each logic gate is formed of p- and n-channel MOS FET's to save power consumption but consumes the power during a short transition period when its electrical state changes to other electrical state. If a great number of logic gates change their electrical states, a large current flows through a power supply wiring, causing a change in voltage at the power supply wiring. Such voltage changes at the power supply wiring becomes large in accordance with a recent trend of increasing the number of logic gates on a single semiconductor chip.
For example, if respective logic gates have a mean load capacitance C.sub.L of 0.2 pF, a switching time tpd of 0.1 nS a power supply voltage of 5 volts, a current Icc flowing through each logic gate can be expressed as EQU Icc=C.sub.L .multidot.Vcc/tpd (1)
and is 10 mA. If, among 100,000 logic gates integrated on a single semiconductor chip, a hundredth of the whole logic gates, i.e. 1,000 logic gates, simultaneously change their electrical states, and instant peak current of 10 A flows through a power supply wiring on the semiconductor chip. Such large current produces a large voltage drop at the power supply wiring. Some of the logic gates malfunction in response to the large voltage drop. In the worst case, the large current generates an electromigration on the power supplying wirings or burns out the power supply wirings.
There has been proposed in Japanese Patent Application Examined Publication No. 49-393 which corresponds to Great Britain Pat. No. 1287110 to mount a concentrated capacitor element on a backsurface of an IC chip for suppressing power voltage change. The capacitor is effective for power supply wirings near a circuit point at which the capacitor is connected. The power supply wirings electrically far from the circuit point, however, still generate a voltage change. Furthermore, the large current still flows through the power supply wiring near the circuit point, because the concentrated element of capacitor only supplies additional current and does not have a function to distributively supply the additional current. The electromigration and the wiring-burnning-out are not prevented by the concentrated capacitor.